How do you know if you’re sending the right information to your subwoofer? It’s a question that’s not immediately obvious to answer. In a well set up system your sub-woofer should blend seamlessly with your mains, and you shouldn’t “notice” it. If your right main speaker stops working, it will clearly not sound correct. If your sub-woofer is only being sent half the bass information it should be sent, how would you know? If your receiver or pre-amp has a sub-woofer output, then it almost definitely has bass management tools that let you dictate what gets sent to your sub. Setting this up right is critical to seamlessly integrating your sub-woofer in your system.
To understand how bass management works, we first need to explore how a receiver handles audio information and how it passes it to your speakers. For the sake of simplicity we are going to talk about a regular home theater receiver, but a pre-amp and separate amplifiers work the same way.
In a 5.1 channel recording (either DTS or Dolby) there are 6 channels of audio information. 5 of these channels are full bandwidth; that is, 20 Hz-20,000 Hz full range frequency information. The sixth channel, the “point one”, is known as the low frequency effects channel and is limited bandwidth. That is, it only includes low frequency information. It is not the “sub-woofer” channel as many people assume. The purpose of the LFE (low frequency effects) channel is to supplement the main 5 channel track with additional low frequency effects. The information that gets encoded into the LFE channel is put there by the sound engineer in the recording studio. The audio from the other 5 full bandwidth channels get sent directly to each of your other speakers.
What is important to note is that unless you have spent a tremendous amount of money on your main speakers, they are not full range speakers. Even large tower speakers with 8″ or 10″ drivers struggle to produce meaningful information south of 40 Hz. More than likely, you have smaller slim towers with low frequency extension in the 45-65 Hz range, and your rear speakers are even higher still. What this means is the audio program has information in the 5 main channels that will be lost. Audio effects might sound “thin” or have poor positioning and panning. What we need to do, is redirect the frequency range that your speakers can not produce, to the subwoofer. This is where the bass management tools come into effect.
The first setting we need to verify is the Speaker Size setting in your receiver. Usually this is Large or Small. You might think that since you bought some nice full size tower speakers that you would want to set this to Large, but this is not the case. These labels should actually be named “Full Range” or “Limited Range”, and as we said before, only the most exotic loudspeakers are truly full range speakers. If you have a speaker set to Large, the receiver thinks that the speaker can handle all the sub bass information encoded in the original full bandwidth channel. If you set the speaker to small, the receiver will apply bass management to that channel and know that it needs to send some of the information to the sub-woofer instead of the main speaker.
After you set all your speaker to “small” the next step is to set up how much of the low frequency information gets sent to the subwoofer. This is done by setting the crossover frequency setting. There are a lot of factors involved in choosing a crossover setting, but speaking in very general terms, a good trade-off is between 60 and 80 Hz. The specs on your speakers might indicate they play lower, but generally they cannot play very low very well and your system is better served by sending this information to your sub-woofer. We will explore crossover settings more in the future. Also note that you can set the crossover different for each speaker, and usually you will have a different setting on your front speakers then the rear speakers.
There are additional settings in most bass management set up menus; usually these are Level and Delay. Level is a setting used to help level match the sub-woofer output to the main speakers. Delay is a setting that adjusts the phase of the sub-woofer to time align the sound output of the sub-woofer so it coincides with the arrival time of the main speakers. This is an advanced topic and will be covered in another article.
So far, we have only talked about multi-channel audio, but the same settings apply to two channel sound, such as that from your CD player or Bluetooth connected device. Without bass management turned on (i.e., speakers set to Large) you will only send audio to your Right Front and Left Front speakers. By ensuring that your speakers are set to small (or limited range, as we like to say) you will send the proper information to your sub-woofer. This greatly relaxes the demands you put on your main speakers which will lower distortion and aid clarity, smoothness, and reduce room modes.
Lastly, with these bass management tools, it’s important to take note of the dials on your actual sub-woofer. Almost all subs have a dial for Crossover, Phase, and Level. When using bass management from your receiver you should have the crossover disabled, or set as high as it can go. Phase is the same as the Delay setting and can be used together with the receiver to properly time align your speakers. The level control is NOT a volume control. Its use to set the gain on the incoming voltage from the receiver or pre-amp.
These tools will help you send the correct information to your sub-woofer to help you get the most from your system. The good news is that most modern receivers have automatic set up tools that use a supplied microphone to set all these settings automatically. Generally they do a great job, but it’s important to double check the settings after it’s complete to make sure they make sense.